1. Field of the Invention
The invention relates to a hybrid vehicle and a control method for the same and, more particularly, a hybrid vehicle that is configured to be capable of executing rotation speed increase control for controlling the rotation speed of an internal combustion engine so that the sound pressure of the internal combustion engine is increased in response to an increase in the vehicle speed, and a control method for the same.
2. Description of Related Art
Engine sound pressure is known as a factor that gives a driver a sense of acceleration during the acceleration of a vehicle. If, engine sound pressure corresponding to an increment in the vehicle speed is not generated, the driver may not feel the vehicle is sufficiently accelerated. Accordingly, techniques for giving a driver a sense of acceleration corresponding to an increment in the vehicle speed by controlling an engine rotation speed so that engine sound pressure is increased in response to an increase in the vehicle speed have been proposed.
For example, in a vehicle traveling control device that is disclosed in Japanese Patent Application Publication No. 2009-210045 (JP 2009-210045 A), an internal combustion engine and a continuously variable transmission are controlled so as to realize a target rotation speed, which is a target value of the rotation speed of the internal combustion engine, in a vehicle on which the continuously variable transmission is mounted. In a case where the vehicle is in an accelerating state, the target rotation speed is set so that an increment in the sound pressure of the internal combustion engine is proportional to an increment in the vehicle speed.
The control of the engine rotation speed for increasing the engine sound pressure in response to an increase in the vehicle speed as described above will be referred to as rotation speed increase control in this specification. Application of the rotation speed increase control to a hybrid vehicle has been examined so as to realize a sense of acceleration in the hybrid vehicle.
The rotation speed increase control will be described in detail later. It is desirable to set a target operating point for an engine to be positioned on an optimal fuel efficiency line regardless of whether or not the rotation speed increase control is executed. In a case where the rotation speed increase control is executed, a target operating point with a low engine rotation speed is set on the optimal fuel efficiency line compared to a case where the rotation speed increase control is not executed. Power (engine output power) that is output from the engine when the engine is driven so as to realize the target operating point is less than engine output power for a case where the rotation speed increase control is not executed. Accordingly, the engine output power may fall short of a power demand for the vehicle (vehicle power demand). In the hybrid vehicle, the power shortfall is supplemented by power output from a motor (motor output power).
Some hybrid vehicles have a charge depleting (CD) mode in which the state of charge (SOC) of the electric power storage device is consumed and a charge sustaining (CS) mode in which the SOC is maintained in a predetermined range as traveling modes. In the CD mode, EV traveling (traveling in a state where the engine is stopped) is mainly performed, and the engine is started in a case where the vehicle power demand cannot be satisfied by the motor output power. In other words, discharging electric power from the electric power storage device (battery) during the start of the engine in the CD mode has a value close to a discharging electric power upper limit value in many cases. Accordingly, if a target operating point with a lower engine rotation speed is set so as to execute the rotation speed increase control, electric power for supplementing the shortfall is unlikely to be supplied from the battery to the motor in a case where the engine output power falls short of the vehicle power demand. As a result, a situation may occur in which it is impossible to satisfy the vehicle power demand.